Digital techniques for audio signal processing extend to the driving of audio output amplifiers. A popular class of amplifier circuits in many audio applications is “class D” amplifiers. A Class D amplifier drives a complementary output signal that is digital in nature, with the output voltage swinging fully from “rail-to-rail” at a duty cycle that varies with the audio information. Complementary metal-oxide-semiconductor (CMOS) output drive transistors are thus suitable for class D amplifiers; as such devices are capable of high, full-rail, switching rates such as desired for digital applications. CMOS drivers conduct low DC current, and their resulting efficiency is especially beneficial in portable and automotive audio applications. In addition, the ability to realize the audio output amplifier in CMOS enables integration of an audio output amplifier with other circuitry in the audio system, further improving efficiency and also reducing manufacturing costs of the system. This integration also provides performance benefits resulting from close device matching between the output devices and the upstream circuits, and from reduced signal attenuation.
As is known in the art, an undesirable effect of a transient signal can exist in the output of a class D amplifier. The transient signal can cause audible artifacts, such as a “click/pop” noise, an audible “humming” noise or a crunching sound. Typically, the transient signals occur when the class D amplifier changes states of operation, such as between an activated state (e.g., switching mode or steady state) and a deactivated state. Reduction (or elimination) of such undesirable transient signals is thusly needed.